When a vessel boundary is penetrated by a relatively rotating shaft, a rotary shaft seal is required to maintain the integrity of the vessel. In a typical piece of rotating machinery, the rolling element bearings require some form of oil bath lubrication, and the bearing housing members form a vessel to contain the lubricant and exclude contaminants. The oil field downhole drilling mud motor sealed bearing sub-assembly is an example of a particularly difficult rotary sealing application. Mud motor seals must perform under a combination of hostile conditions including high differentiaI pressure across the seal, (as much as 1500 psi), high fluctuating levels of lateral shaft deflection, elevated temperature environment, geothermal heat and heat generated by bearings and seals, highly abrasive drilling fluid environment, limited lubricant reservoir volume, static shaft-to-housing misalignment, high levels of vibration, and axial shaft motion due to internal clearances and component elasticity. A viable mud motor seal implementation must have the ability to continue to perform its sealing function under the aforementioned hostile combination of conditions with low leakage and a long dependable service life. The mud motor, which is positioned at the bottom end of a drillstring during well drilling operations, is a positive displacement hydraulic motor that induces rotation of the drill bit against the earth formation being drilled. Mud motors are driven by the recirculating drilling fluid, which also functions to cool the drill bit and to flush drilling cuttings out of the well bore.
In high pressure abrasive liquid environment installations, such as in downhole mud motors, elastomeric shaft seals perform better if the extrusion gap between the shaft and the seal carrier is relatively small. If the extrusion gap is too large, the elastomeric seal material tends to bulge into the extrusion gap between the shaft and seal carrier due to the differential pressure between the internal lubricant and the external drilling fluid. Lateral movement of the shaft (due to side loads, runout, and bearing clearances) tends to pinch or "nibble" away at any seal material which protrudes into the gap; the resulting loss of seal material tends to shorten seal life. In practice the extrusion gap must be as small as practical to avoid extrusion nibbling of the material, but it must also be sufficiently large to avoid contact with the shaft. If the extrusion gap is sufficiently small that the seal carrier contacts the shaft, the seal carrier will assume part of the side load intended for the bearings and the resulting heat and friction will damage the elastomeric seal, the sear carrier, and the shaft. Seal carriers are usually integral to the bearing housing, or rigidly mounted to the housing. With such designs, the conflicting requirements (minimizing the extrusion gap while simultaneously avoiding shaft contact) require the maintenance of undesirably tight component tolerances.
A complete mud motor consists of three principal sub-assemblies: a hydraulic motor, a universal joint (U-joint), and a bearing assembly. Motor operation is relatively simple in principle concept. Circulating drilling fluid turns the rotor of the hydraulic motor. The U-joint transfers rotary motion from the rotor of the hydraulic motor to the bearing sub-assembly rotary shaft, to which the drill bit is threadedly connected. Weight is transferred from the drillstring to the drill bit via the thrust bearings of the bearing sub-assembly. As the drill bit rotates, it bears against and fractures the geologic formation by virtue of the weight on the bit, which is concentrated for that purpose by the cutting structure of the bit. The radial bearings of the bearing sub-assembly serve to orient and guide the bit with respect to the drillstring. In steerable drilling systems, a bent mud motor housing is used between the motor sub-assembly and the bearing sub-assembly. Directional control is accomplished by turning the drillstring momentarily to point the bent or angulated housing in the desired direction of travel. Straight ahead drilling is accomplished by continuous rotation of the drillstring. Bent housings place additional side loads on the bearing sub-assembly radial bearings during both the straight and the directional drilling modes, and in cooperation with high frequency variations in axial loading, contribute significantly to the high levels of fluctuating shaft deflection.
Seals are the weakest link in the mud motor sealed bearing sub-assemblies currently being used; the majority of bearing failures are preceded by failure of the lubricant seal which is intended to maintain the lubricant inside the sealed assembly and to prevent entry of abrasive contaminants into the lubricant chamber of the motor.
Many present-day mud motors still use unsealed, mud lubricated bearing assemblies because of the difficulties associated with successfully implementing a rotary seal which is capable of withstanding the unusually hostile conditions of the downhole drilling environment.
In unsealed bearing assemblies, the radial loads from the bit and the universal joint are carried by elastomer marine bearings, and the axial loads are carried by a custom stack of ball thrust bearings, often made from tungsten carbide to help resist the abrasive effects of the drilling fluid. Both the radial and the thrust bearings are cooled and lubricated by the diversion of a small portion of the circulating drilling fluid. Unsealed bearing sub-assemblies have a severely limited operating life, typically in the range of 50 to 100 hours of drilling activity, owing to baring wear. This limitation inflates drilling costs by requiring frequent trips out of the hole for motor replacement. Relatively high custom bearing replacement costs also add to the overall drilling costs. A substantial monetary savings could be realized if trips in and out of the hole for motor replacement were less frequent. Reduced motor maintenance costs are also desirable. These goals are starting to be realized by sealed bearing sub-assemblies which use conventional off-the-shelf bearings in a clean, lubricated operating environment.
Several types of radially compressed ring-shaped squeeze packing type seals are currently used to seal mud motor bearing assemblies. The most notable example is the patented, hydrodynamically lubricated, elastomeric shaft seals which are manufactured and sold by Kalsi Engineering, Inc., Sugar Land, Tex., under the registered trademark KALSI SEAL.RTM.) Shaft seals manufactured by Parker and sold under the registered trademark PolyPak.RTM. have also been employed, but with less success. At the present time, certain carefully implemented hydrodynamic seal installations in mud motors are routinely providing motor run times in excess of 160 hours with seals still effective; in other less carefully implemented installations seal failure may occur sooner. In a high pressure above ground Kalsi Seals shaft seal application where runout and shaft to housing clearances could be minimized, seal life in excess of 1000 hours has been reported from the field. The inner surface of a hydrodynamic Kalsi Seal type shaft seal has a unique, patented geometry that promotes increased seal life by lubricating the dynamic seal-to-shaft interfacial zone and by excluding drilling environment abrasives from the shaft sealing interface. The patented geometry incorporates a wavy, axially varying edge on the lubricant side of the inner diameter and a straight edge on the environmental side. As relative rotation takes place, the wave shape on the lubricant side, which has a gradually converging shape in the axial direction, generates a hydrodynamic wedging action that introduces a minute lubricant film between the seal and the shaft. This film physically separates the seal and the shaft, which prevents the typical dry rubbing type of wear and heat generation associated with a conventional non-hydrodynamic squeeze packing type seal and thereby prolongs seal and mating shaft surface life. The straight edge on the environmental side of the hydrodynamic seal identified by applicant's registered trademark Kalsi Seals is sharp cornered and does not generate a wedging action, and thereby helps to exclude particulate contaminants from the seal-to-shaft interface.
In conventional mud motor sealed bearing sub-assembly design, squeeze packing type seals, including Kalsi Seals.RTM. type hydrodynamic shaft seals, are installed in a groove cut into the bore of a thru-hole provided for the shaft penetration of a housing member. In some cases the groove and thru-hole are integral with one of the bearing housings; in other cases they are part of a separate seal carrier housing that is rigidly mounted within the bearing housing. The groove diameter is sized so that the resilient sealing element is held in radial compression against the outside of the shaft. The resulting contact pressure initiates a seal between the resilient sealing element and its mating surfaces on the seal groove and shaft, and thereby maintains the integrity of the lubricant vessel.
All mud motor sealed bearing assembly housings are filled with bearing lubricant which is pressure balanced to the
drilling fluid pressure in the drillstring bore by means of a pressure transmitting partitioning device such as a free-floating piston. The lubricant is retained within the housing by means of rotary sealing elements at each end of the housing. As the drilling fluid passes through the drill bit jets and enters the annulus of the well, its pressure drops to a level which is approximately 500 to 1,500 psi below the drillstring bore pressure. The bearing assembly seal arrangement therefore must withstand a 500 to 1,500 psi pressure drop between the bearing lubricant and the drilling fluid in the well annulus.
It is widely known that elastomeric squeeze type packings require a relatively small shaft-to-housing clearance gap in order to perform satisfactorily at elevated pressures, such as those found in typical mud motor operating conditions. Differential pressure across an elastomeric sealing element drives it against the wall on the low pressure side of the housing groove. As pressure increases, the elastomer tends to bulge into the shaft-to-housing clearance gap. The tendency toward protrusion is a direct function of the operating pressure differential and the size of the shaft-to-housing gap; larger gaps cause increased bulging. In extreme conditions, the seal material extrudes completely through the shaft-to-housing clearance gap by direct shearing action. Continuing loss of seal material ultimately leads to seal failure. The force which drives the shearing action is the differential pressure acting over the portion of the seal that is not supported by the seal groove wall. The key factors affecting direct extrusion are: shaft-to-housing clearance, differential pressure, and elastomer hardness. In less extreme conditions, the seal protrudes into the shaft-to-housing clearance gap, but does not fail by direct extrusion. Instead, cyclic strain, which results from normal pressure fluctuations and/or fluctuating extrusion gap size, causes the protruding material to break away from the sealing element. When this type of damage occurs, the damaged portion of the seal appears as if it has been nibbled away. This type of damage is a relatively common occurrence when elastomeric seals are used in high pressure rotary shaft applications.
For long-term moderate temperature 1,500 psi static (non-rotary) sealing applications using 90 durometer Shore A materials, various squeeze packing manufacturers recommend a maximum radial shaft-to-housing gap of 0.007- to 0.008-inch after taking into account tolerances, shaft-to-housing misalignment, and pressure distortion of the mechanical components. Smaller gaps are recommended for elevated temperatures. Several squeeze packing manufacturers give O-ring and X-ring installation recommendations for pressurized rotary applications The shaft-to-housing clearance recommendations for rotary applications are smaller than for static applications, presumably because of unavoidable elastomer softening resulting from seal generated heat. When rotation is present, the localized temperature at the rotary seal to shaft interface is always significantly higher than the ambient environment. In non-hydrodynamic squeeze packing shaft seals the seal generated heat is the result of direct seal to shaft rubbing, and is so severe that it can cause the elastomer to blister and melt. The self generated heat associated with hydrodynamic seals is less severe; but can still result in significant softening of the seal material, and consequently lower the pressure retaining ability of the seal. The heat generated by hydrodynamic seals is the result of shearing of the hydrodynamic lubricant film in the seal to shaft interfacial zone, and is not the result of heat generated by direct seal to shaft contact. Nevertheless, even with hydrodynamic seals, elastomer softening at the interface dictates that extremely close clearances be maintained in pressurized applications. The problem becomes more acute as rotational speeds and pressures are increased. In actual practice, mud motor rotating shafts are not precisely concentric to their housings, and in normal present day practice, shaft-to-housing clearances have to be adjusted accordingly so that there is sufficient clearance to prevent metal to metal contact between the shaft and the seal carrier. Mud motor shafts are prone to static misalignment with respect to the housing, and also to relatively large fluctuating lateral motions. The static misalignment results from diametric assembly clearances, non-perpendicular mounting shoulders, and eccentric mounting diameters resulting from normal tolerances. The dynamic lateral motion results from shaft deflection under fluctuating side loads, articulation within normal operating and assembly clearances, eccentricity and out-of-roundness due to normal production machining practices, and radial bearing stiffness under side loads. It is widely known that rotary shafts should not be permitted to rub their respective housings in squeeze packing seal arrangements, because the resulting friction can cause seal damage due to heat build-up as well as shaft and housing damage.
Detailed investigations by the inventor and his associates, which included finite element analysis, tolerance analysis, and trigonometric articulation analysis, show that mud motor shaft misalignment and dynamic lateral motion in the vicinity of the squeeze packings ordinarily exceeds standard industry recommendations for maximum radial shaft-to-housing clearance gaps for static, moderate temperature, 1,500 psi squeeze packing applications. This means that if the clearance is designed sufficiently large enough to prevent shaft-to-housing rubbing, the eccentric gap will be significantly greater than the industry recommendations for elevated temperatures.
Two distinct types of squeeze packing seal installations are currently being used, one which avoids metal to metal contact between the shaft and seal carrier by adjusting the size of the housing thru-bore, and one which permits such metal-to-metal contact. Many currently operational squeeze packing type sealed bearing mud motor sub-assemblies deliberately permit shaft-to-housing contact in order to maintain a relatively small shaft-to-housing clearance gap. The contact typically occurs at a bushing, which also defines the shaft-to-housing clearance in the vicinity of the seal.
When lateral shaft motions exceed the bushing-to-shaft clearance, the bushing assumes side loads that would preferably be borne by the rolling element bearings. As a result, a local build-up of heat occurs which is detrimental to the seal. The modulus of elasticity of the seal is lowered, which in turn lowers the pressure capability of the seal. The heat build-up also speeds up the compression set of the elastomer, thereby shortening the life of the seal. Local melting of the seal can also occur. The heavily loaded metal to metal rubbing contact also results in wear of the shaft and housing, and so increases the size of the shaft to housing clearance with direct detrimental results to the pressure bridging capacity of the sealing element. The resulting shaft wear is in the form of a localized, rough surfaced groove, with material deposits transferred from the housing bushing.
When relative axial motion occurs between the housing and the shaft as the result of internal assembly clearances and elasticity of the supporting components, the leading edge of the seal can become damaged from riding over the edge and roughened bottom surface of the groove. In some cases the combination of negative effects associated with this type of design can lead to premature failure. The alternate present day type of seal implementation, which is intended to prevent the problems associated with the aforementioned metal to metal contact, uses clearances which are substantially greater than the standard industry recommendations for static, moderate temperature 1500 psi applications. These relatively large clearances, in concert with relative motion caused by lateral shaft deflections, promote nibbling type damage to the shaft seal. At any given instant of operation, the clearance at a particular location is quite large and the elastomer readily protrudes into the gap; at the next instant, the gap closes and the protruding material is heavily compressed. The cyclic compression and relaxation eventually breaks off the protruding material and at the same time contributes to localized heat build-up, which exacerbates the extrusion problem. Therefore present day squeeze packing seal implementations which avoid metal to metal contact are limited in use to relatively low pressure differential mud motor applications. The current designs of the type which are intended to prevent shaft to carrier rubbing contact also attempt to minimize shaft misalignment by employing severe reductions of manufacturing tolerances and assembly clearances. This results in undesirably high manufacturing costs and unreasonably difficult assembly, but it still does not permit the type of minimum shaft-to-housing clearances that support the desired seal life at the upper limit of differential pressures encountered in drilling mud motors. Such designs also place the sealing element as close as possible to the rolling element bearings in order to minimize the lateral motion associated with articulation and deflection due to shaft overhang past the bearings. All bearings generate heat, however, and some sources recommend against placing squeeze type packings in close proximity to radial bearings for that reason.
Some published literature recommends "floating" seal housings for rotary applications when static shaft misalignment exceeds the shaft-to-housing clearance. The recommended floating housings consist of a ring which incorporates the rotary squeeze packing in a bore groove and a larger diameter static squeeze packing between the outside diameter of the floating seal carrier and the inside diameter of the bearing housing. This arrangement only "floats" to a very limited degree in pressurized applications, because once differential pressure occurs, the piston is forcibly held against its retainer by the axially acting hydraulically induced force which results from differential pressure acting over the hydraulic area between the rotary seal on the inside and the static seal on the outside of the carrier. The resulting friction between the carrier and its retainer greatly inhibits the ability of the carrier to float freely. Such designs are incapable of floating freely once differential pressure is applied, and can only float effectively in response to static misalignment prior to pressure introduction. Even that meager benefit is more or less negated by the fact that compression of the static seal on the outside diameter has a strong tendency to counteract any centering effect provided by compression of the rotary seal. The static seal is generally compressed to a higher degree than the rotary seal, and also has a larger circumference in compression, and therefore plays a correspondingly greater positioning role than the rotary seal. The lateral motion of the shaft has other detrimental effects on seal performance aside from the ones associated with shaft-to-housing clearance.
In well controlled high rotary speed, low pressure sealing tests, it has been repeatedly demonstrated that an elastomer sealing element cannot rebound quickly enough in response to minor shaft runout to prevent increased leakage. In recent controlled rotary seal experiments, it has been demonstrated that the leakage rate of high pressure elastomeric seals can sometimes increase dramatically with increased levels of shaft runout. It appears that the high friction of the elastomer, as it is forced against the low pressure gland wall by the high differential pressure, inhibits elastic rebound and causes undesirably high leakage rates. Lateral shaft motion of a similar or greater magnitude and frequency is to be expected in mud motor seal installations. These tests were performed in the absence of a drilling fluid operating environment; abrasive ingestion is distinctly possible when elastomeric rebound is inhibited, especially when dynamic axial shaft motion is simultaneously present.
Another problem related to mud motor shaft to housing lateral motion and misalignment involves seal compression. When a condition of eccentricity exists between the shaft and the seal gland, the radial seal compression decreases over approximately 1/2 of the seal circumference and increases on the opposite half. In order to insure that a fluid tight seal is maintained under such eccentric conditions, sufficient preliminary compression must be provided so that a sufficient level of compression is ensured in the offset condition. This usually results in a relatively high initial compression requirement when the lateral motion of mud motor shafts is being considered. Finite element analysis of the standard cross-section Kalsi Seal type hydrodynamic shaft seal under various levels of elevated temperature and compression indicate that distortion associated with high compression is undesirable. Increasingly high seal-to-shaft contact pressures are also associated with higher compression levels which result in undesirable increases in seal generated frictional heat and in running and startup torque.
Yet another problem related to mud motor shaft to housing lateral misalignment and motion involves wear of the bore and shaft in the close fitting region which defines the clearance between the shaft sealing surface and the housing member. The region is subjected to immersion drilling fluids which contain abrasive particulate matter, such as bentonite and cuttings broken from the formation by the drilling operation The drilling fluid, typically known as drilling mud, utilizes water and other liquid materials as a carrier constituent for the highly abrasive solid particulate matter. Due to the high ambient environmental pressures, and the constant relative motion, these abrasives are continually present in the closely fitting gap between the shaft and the seal carrier. When shaft lateral motion occurs, the shaft to housing clearance is reduced over approximately 1/2 of the circumference of the shaft, and as a result abrasives are trapped and crushed between the approaching surfaces of the shaft and housing. This crushing action causes serious wear to both the housing and the shaft, even though the shaft is usually coated with a hard surfacing material such as tungsten carbide. The housing wear causes the housing bore diameter to become larger which results in increased seal extrusion damage. The shaft wear is in the form of a localized, rough surfaced groove. When relative axial motion occurs between the housing and the shaft as the result of internal assembly clearances and elasticity of the supporting components, the leading edge of the seal can become damaged from riding over the edge and roughened bottom of the groove.
The present invention involves a seal implementation which successfully overcomes the problems enumerated above for an oil field downhole drilling mud motor high pressure seal, and provides a highly desirable squeeze packing seal implementation for many other types of difficult rotary sealing applications as well.